On a Pool of Tin: Float Glass

All About Glass

The auto and building industries boomed after World War II. Plate glass was in extremely short supply. Twin-grinding—used all over the world to make the glass—was too costly and too slow to keep up with the demand. Could high-quality glass be made without grinding and polishing?

Alastair Pilkington, an engineer at Britain’s Pilkington Brothers, was washing dishes when the idea struck: maybe it is possible to form a perfectly flat sheet by floating molten glass on a pool of liquid tin. That way, the glass would be smoothed by gravity and surface tension, rather than being squeezed by rollers. If the glass wasn’t marred by rollers, it wouldn’t need grinding and polishing.

The idea was brilliant in its simplicity, but seven costly years were needed to make it work. The risk paid off. The revolutionary Pilkington float process, introduced in 1959, is used today to make nearly all the flat glass in the world.

Making float glass

A float glass factory is tireless. Its tank can operate continuously for up to 12 years—making over 50,000 miles (80,000 kilometers) of high-quality flat glass almost automatically.

The first step in making float glass occurs in the melting tank. Pre-mixed raw materials continually move into a furnace, where they are melted. From the melting tank, the molten glass flows into the float tank by passing over a spout landing onto a shallow pool of liquid tin. Here, surface tension and gravity distribute the viscous molten glass into a relatively flat ribbon of uniform thickness, while edge rollers maintain the desired width. From the float tank, the glass continues rolling into the annealer where the solidified ribbon is cooled slowly to prevent stresses from building up in the glass. As the ribbon moves along, it further cools and hardens. Once the glass is through the cool-down phase, the edges of the ribbon, marred by the rollers, are trimmed off, and the glass is cut to size. At the end of the process, robotic arms lift the glass panes and load them on a storage rack.

The Corning Museum of Glass
This article was originally published in Innovations in Glass, 1999, pp. 14–15.